When Michael J. Sullivan was acting director of the Bureau of Alcohol, Tobacco, Firearms, and Explosives, he oversaw Project Gunrunner, aimed at stemming illegal gun trafficking to Mexico.

By the time an element of the program known as Operation Fast and Furious exploded into controversy in 2011, Sullivan was working the issue from the private sector, as a lawyer hired by a gun industry group to dissuade the Mexican government from filing suit in the United States over illegal guns flowing into Mexico.

The dual roles highlight Sullivan’s sometimes confusing profiles as a tough-on-crime prosecutor and a gun rights advocate. The former prosecutor and ATF acting director is the only one of the three Republicans running for US Senate in Tuesday’s primary who said he would have voted against the gun compromise the Senate considered this month.

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Sullivan, through campaign manager Paul Moore, refused to provide ­details of his involvement in fending off the threatened litigation.

“Mike’s work on behalf of his clients isn’t something he can speak about publicly,” Moore said. “At no time has he done work for any client in opposition to the mission of ATF,” Moore added.

Moore said that Sullivan aggres­sively enforced laws against illegal gun trafficking both as US attorney and as acting ATF director.

“They don’t call him ­Maximum Mike for nothing; he was very vigorous in doing that,” Moore said. “On the other hand, sportsman’s groups and affiliated interests serve law-abiding gun owners, and that’s a very different group of people involved.”

Sullivan was appointed acting director of the ATF by President
President George W. Bush in late 2006 and left with the start of the Obama administration in 2009.

Under Sullivan, the agency had been working to stop gun violence on both sides of the border, using firearms tracing to identify trafficking trends. But the program was later criticized for allowing individual gun smugglers to continue to send weapons into Mexico while federal agents gathered evidence and tried to build cases against a broader network. A controversy erupted after the 2010 death of a Border Patrol agent in an Arizona shoot-out was traced back to guns linked to Operation Fast and Furious.

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When the federal government refused to release documents on the operation, Congress found Attorney General Eric Holder in contempt and sued the Department of Justice to obtain documents.

In 2010, the Mexican government hired a New York law firm, Reid Collins & Tsai, to pursue litigation against unspecified US defendants
, citing harm being done by illegal weapons in Mexico.

The suit, which was never filed, aimed to target American sources of guns for the illegal trafficking across the border. Conservatives argued that the Mexican government was trying to shift blame for its own cartel violence to the United States.

The National Shooting Sports Foundation issued a statement in April 2011 saying that it “respects the work of [President Felipe Calderón of Mexico] to willingly take on his country’s powerful drug cartels. However, we are disappointed that he would seek to hold law-abiding American companies responsible for crime in ­Mexico.”

The foundation confirmed earlier this month that it hired Sullivan to do legal work, rather than lobbying, and that his work included the threatened litigation from Mexico.

“In 2011, the Mexican government threatened to file a lawsuit in the United Sates against members of the firearms industry alleging a legally baseless claim that members of the industry were somehow liable for the criminal misuse of firearms in Mexico,” the foundation’s general counsel Larry Keane said in that statement. “Mr. Sullivan assisted NSSF in responding to that threatened litigation including engaging in discussions with officials of the Mexican government.”

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The National Shooting Sports Foundation is located in Newtown, Conn., the site of the December elementary school shootings that galvanized the White House’s unsuccessful push for increased gun control.

Sullivan has been outspoken in favor of gun rights throughout a campaign that has often focused on the issue of gun violence. Both Gabriel E. Gomez and state Representative Daniel B. Winslow, his Republican ­rivals, said they would have backed a compromise measure on gun control that would have eliminated the “gun show loophole” that allows gun sales from gun shows without background checks in some states.

In an interview on WGBH Thursday, Sullivan dismissed the notion of a “gun show loophole,” and any connection to crime, saying, “Criminals don’t pick up guns at gun shows. They get them at other places.”

Asked whether the Boston Marathon bombing suspects’ shootout with police last week suggested a need to limit magazine capacity or the assault weapons civilians can buy, ­Sullivan said no.

“The people involved in these horrific bombings . . . they wouldn’t pay atten­tion no matter which type of rules and regulations you’d put in place,” he said.